Polyether polyols prepared using double metal cyanide (DMC) catalysts such as zinc hexacyanocobaltate are well known. (See, for example, U.S. Pat. Nos. 3,278,457 and 4,477,589; R. J. Herold and R. A. Livigni, Adv. in Chem. Series: Polymerization Kinetics and Technology, No. 128 (1973) 208; J. L. Schuchardt and S. D. Harper, Proceedings of the SPI--32nd Annual Polyurethane Technical/Marketing Conference, Oct. 1989, p. 360). A key advantage of these polyols compared with those prepared with a conventional (KOH) catalyst is the high functionality--even at high equivalent weight--and low unsaturation of polyols made with DMC catalysts.
Removal of double metal cyanide catalyst residues from polyether polyols promotes long-term storage stability and consistent polyol performance in urethane formulation. Polyols that contain catalyst residues generate volatile impurities that give the polyol an undesirable odor. In addition, the catalyst residues often catalyze undesired reactions during polyurethane formulation.
Unfortunately, DMC catalyst residues are often troublesome to remove from polyether polyols. Typical catalyst removal methods for conventional base catalysts--such as heating the polyol with magnesium silicate, passing the polyol through an ion-exchange resin, or acid-treating the polyol followed by ion-exchange treatment--are generally ineffective for removing DMC catalyst residues. More rigorous methods include heating the catalyst-containing polyol with alkali metals, alkali metal hydroxides, or alkali metal hydrides (see U.S. Pat. Nos. 4,355,188 and 4,721,818). Unfortunately, these methods are often slow, use expensive, toxic, and/or highly reactive reagents, introduce unwanted color into the polyol, and give inconsistent catalyst removal performance. In another process (U.S. Pat. No. 5,010,047), DMC catalyst residues are removed by diluting the polyol with a nonpolar solvent such as hexanes or toluene, followed by filtration. This process uses a large amount of solvent and filter aid.
Watabe et al. (U.S. Pat. No. 4,987,271) teach a method for purifying polyether polyols prepared using DMC catalysts. The method comprises heating the catalyst-containing polyol with a pH buffer solution, then (optionally) adding a chelating agent to the mixture, adding an adsorbent (magnesium silicate, alumina, etc.) or ion-exchange resin, and filtering the mixture. When a chelating agent is used, ammonia can be used instead of a pH buffer.
Another known process for removing DMC catalyst residues from polyether polyols involves (a) heating a polyol that contains DMC catalyst residues with 88:12 (w/w) isopropanol/water (1 part of polyol to 2 parts of isopropanol/water) at 80.degree. C. for 8 h in the presence of 0.5 weight percent of dipotassium EDTA; (b) filtering the resulting EDTA complex from the polyol; (c) passing the polyol solution through a mixed anion/cation ion-exchange resin bed; and (d) stripping the isopropanol/water away from the polyol. This process is rather impractical commercially because of the large volume of solvent that must be stored, transferred, and stripped, and the need for ion-exchange facilities. The method suffers from one additional important drawback: potassium ions introduced into the polyol with the dipotassium EDTA are difficult to remove when 2 parts of isopropanol/water are used for each part of polyol unless the ion-exchange treatment is included.
Still needed in the art is a reliable, practical process for removing DMC catalysts from polyether polyols. A preferred process would avoid pre-treatment of the polyol with toxic, reactive reagents, and would eliminate the need for adsorbents, which add to raw material and waste disposal costs. A preferred process would use a minimum amount of solvent, and would allow removal of double metal cyanide catalyst residues without introducing additional cationic moieties. A preferred process would eliminate the need for ion-exchange treatments.